Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR), in combination with CRISPR associated genes (cas) constitute the CRISPR-Cas system, which confers adaptive immunity in many bacteria and most archaea. CRISPR-mediated immunization occurs through the integration of DNA from invasive genetic elements such as plasmids and phages that can be used to thwart future infections by invaders containing the same sequence.
CRISPR-Cas systems consist of CRISPR arrays of short DNA “repeats” interspaced by hypervariable “spacer” sequences and a set of flanking cas genes. The system acts by providing adaptive immunity against invasive genetic elements such as phage and plasmids through the sequence-specific targeting and interference of foreign nucleic acids (Barrangou et al. 2007. Science 315:1709-1712; Brouns et al. 2008. Science 321:960-4; Horvath and Barrangou. 2C10. Science 327:167-70; Marraffini and Sontheimer. 2008. Science. 322:1843-1845; Bhaya et al. 2011. Annu. Rev. Genet. 45:273-297; Terns and Terns 2011. Curr. Opin. Microbiol. 14:321-327; Westra et al. 2012 Annu. Rev. Genet. 46:311-339; Barrangou R. 2013. RNA. 4:267-278). Typically, invasive DNA sequences are acquired as novel “spacers” (Barrangou et al. 2007. Science. 315:1709-1712), each paired with a CRISPR repeat and inserted as a novel repeat-spacer unit in the CRISPR locus. The “spacers” are acquired by the Cas1 and Cas2 proteins that are universal to all CRISPR-Cas systems (Makarova et al. 2011. Nature Rev. Microbiol. 9:467-477; Yosef et al. 2012. Nucleic Acids Res. 40:5569-5576), with involvement by the Cas4 protein in some systems (Plagens et al. 2012. J. Bact. 194: 2491-2500; Zhang et al. 2012. PLoS One 7:e47232). The resulting repeat-spacer array is transcribed as a long pre-CRISPR RNA (pre-crRNA) (Brouns et al. 2008. Science 321:960-4), which is processed into CRISPR RNAs (crRNAs) that drive sequence-specific recognition of DNA or RNA. Specifically, crRNAs guide nucleases towards complementary targets for sequence-specific nucleic acid cleavage mediated by Cas endonucleases (Garneau et al. 2010. Nature 468:67-71; Haurwitz et al. 2010 Science 329:1355-1358; Sapranauskas et al. 2011. Nucleic Acid Res. 39:9275-9282; Jinek et al. 2012. Science. 337:816-821; Gasiunas et al. 2012. Proc. Natl. Acad. Sci. 109:E2579-E2586; Magadan et al. 2012. PLoS One. 7:e40913; Karvelis et al. 2013. RNA Biol. 10:841-851).
These widespread systems occur in nearly half of bacteria (˜46%) and the large majority of archaea (˜90%). CRISPR/Cas are subdivided in classes and types based on the cas gene content, organization and variation in the biochemical processes that drive crRNA biogenesis, and Cas protein complexes that mediate target recognition and cleavage. Class 1 uses multiple Cas proteins in a cascade complex to degrade nucleic acids (see, FIG. 1). Class 2 uses a single large Cas protein to degrade nucleic acids. The type I systems are the most prevalent in bacteria and in archaea (Makarova et al. 2011. Nature Rev. Microbiol. 9:467-477) and target DNA (Brouns et al. 2008. Science 321:960-4). A complex of 3-8 Cas proteins called the CRISPR associated complex for antiviral defense (Cascade) processes the pre-crRNAs (Brouns et al. 2008. Science 321:960-4), retaining the crRNA to recognize DNA sequences called “protospacers” that are complementary to the spacer portion of the crRNA. Aside from complementarity between the crRNA spacer and the protospacer, targeting requires a protospacer-adjacent motif (PAM) located at the 5′ end of the protospacer (Mojica et al. 2009. Microbiology 155:733-740; Sorek et al. 2013. Ann. Rev. Biochem. 82:237-266). For type I systems, the PAM is directly recognized by Cascade (Sashital et al. 2012. Mol. Cell 46:606-615; Westra et al. 2012. Mol. Cell 46:595-605). The exact PAM sequence that is required can vary between different type I systems. Once a protospacer is recognized, Cascade generally recruits the endonuclease Cas3, which cleaves and degrades the target DNA (Sinkunas et al. 2011. EMBO J. 30:1335-1342; Sinkunas et al. 2013. EMBO J. 32:385-394).